seed library?

While on vacation in the US I read a gardening magazine that had an interesting article about seed libraries. People have been starting seed libraries in real libraries...there's a section in the library that has seeds, you can "check-out" a packet, grow it, and then you're asked to return a packet of the seeds at the end of the growing season...

I'd like to do something like this. It won't work in my local library, but I was thinking of doing something online where I would take my excess seeds, package them, list them, and then people could contact me about them. For a SAE I would send them a packet of seeds and ask for a packet of the same variety back that they've saved at the end of the season. It wouldn't be like swapping because I wouldn't be exchanging one variety for another. I know there are larger places that do something similar (like the HSL - which I am a member of), but this would be small scale (and I think any exchange of heritage seeds is a good thing).

What do you think? Any pitfalls I should be aware of?